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Herein, the effect of metal contact doping on the scaled graphene field effect transistor (GFET) is investigated. Different from the traditional semiconductors device, the drain current of GFET is not inversely proportional to the channel length (LCH). The abnormal scaling behavior for drain current in GFETs can be attributed to the modification of channel resistance induced by the penetration of contact metal doping. In addition, the field‐effect mobility (μEF) of long channel GFET trend to saturate with decreasing LCH, which is consistent with the diffusive transport model. As the channel length is further scaled down, the μEF at first increases drastically and then decreases due to the enhanced effect of electrical property of graphene under the metal electrode as well as the contact resistance on the carrier transport of GFET. This study indicates that there will be a trade‐off between scaled channel length and the best performance of GFET. Further efforts should be made to modulate the properties of graphene both in channel and contact region to improve the scaling behavior of GFET. [ABSTRACT FROM AUTHOR]

Graphene has great application potential in radio frequency (RF) electronics such as low-noise amplifier (LNA) due to its amazing electrical properties. In this work, a graphene-transfer after process was developed to fabricate two-stage graphene LNA monolithic microwave integrated circuits (MMICs) on sapphire substrate to reduce possible damage to the graphene material in the device fabrication process. Using microstrip lines, inductances, and capacitances as input–output, and interstage impedance match networks, the two-stage Ku-band graphene LNA MMIC was obtained with a 10.2 dB maximum gain at 12.7 GHz, and 4.2 dB minimum noise figure at 12.5 GHz. [ABSTRACT FROM AUTHOR]

The most attractive application of graphene in the RF area is low-noise amplifier (LNA) due to its amazing properties. But, design and fabrication of graphene LNA Monolithic Microwave Integrated Circuits (MMICs) are still blank. In this work, 200 nm gate-length chemical vapor deposition (CVD) monolayer graphene transistors were fabricated on a sapphire substrate. The graphene transistor shows an intrinsic gain of ${g} _{\text{m}}/{g} _{\text{ds}}= {1.1}$ , and high extrinsic maximum oscillation frequency with ${f} _{\text{max}}$ (48 GHz) > cutoff frequency ${f} _{\text{T}}$ (43 GHz). The graphene transistors show low extrinsic minimum noise figure of 2–7 dB in the measured frequency range of 6–50 GHz. A C-band graphene LNA MMIC was designed and fabricated and shows a maximum gain of 8.34 dB at 5.5 GHz, and minimum noise figure of 4.96 dB at 5.8 GHz. This work demonstrates the application potential of graphene for future RF high-speed electronics. [ABSTRACT FROM AUTHOR]

Modulating the electronic property of graphene by doping is essential for its device and circuit applications. Unfortunately, controllable p‐ and n‐type doping in top‐gated graphene field effect transistor (GFET) is reported. Here, the O3‐based atomic layer deposited Al2O3 layer as top gate dielectric is chosen. The epoxide functional group formed in the O3 process serves as effective interface trap sites. As the sweeping range of top gate voltage increases, the Dirac point position of GFET moves from positive voltage to negative voltage. The shift of the Dirac point voltage indicates the doping transition of graphene from p‐type to n‐type. The gate voltage dependent doping can be attributed to the charge exchange between graphene and interface trap sites. Furthermore, a trap‐dependent charge model is proposed to explain the transport mechanism in the doping process. This approach is promising to produce the complementary p‐ and n‐type top‐gated GFET for multiple applications. [ABSTRACT FROM AUTHOR]

The rarely reported defects with crack-and-fold (CAF) style graphene grown on raw Cu foils substrates by chemical vapor deposition (CVD) were examined in this study. Factors for the formation of CAF defects were explored by performing different substrate treatments and varied hydrogen inflow. A clear relationship was found between CAF defects and Cu (110) facets by electron back-scattered diffraction measurement. CAF defects were affected by the interaction between graphene and the substrate. A model with a two-step process, including cracking and folding, is established to elucidate the formation of CAF defects. CAF defects can be reduced by performing substrate treatment, varying hydrogen flow and controlling the orientation of the Cu substrate. This study was performed to improve the quality of graphene grown by CVD for industrial production. [ABSTRACT FROM AUTHOR]

Abstract: The Dirac point(s) in graphene field‐effect transistors (GFETs) are of great importance for electronic application. However, the lack of the effective means to distinguish the electrical properties of graphene at the contact and channel regions limits a clear understanding of their contributions to the Dirac point(s). A method, which can characterize the electrical properties of graphene under metal contact and in the channel, is developed, respectively. It is found that the Fermi levels of graphene at the contact and channel regions are quite different in the GFETs. The difference in Fermi levels results in the penetration of the doping effect under the contact into the channel with a length as long as 1 µm. The difference also causes the double Dirac point feature in the long‐channel GFET due to the combined graphene properties both in contact and channel. One of the two Dirac points diminished in short‐channel GFET due to the penetration of doping effect under the contact. The study demonstrates that the electrical behavior of short‐channel GFET is dominated by the contact region. This paces a way to deeply understand and further improve the performances of GFETs by controlling the Fermi levels in the whole devices. [ABSTRACT FROM AUTHOR]

Reconfigurable artificial synapse with synaptic responses modulated between excitatory and inhibitory modes is critical for building artificial intelligence systems. However, it is still a challenge to realize such reconfigurability with a simple single‐gated transistor. Here, hydrogen‐rich silicon nitride film is employed as the gate dielectric to construct a single‐gate controlled graphene‐based artificial synapse to realize the reconfigurable synaptic responses. In this dielectric, both traps and movable hydrogen ions are introduced to induce the carrier trapping effect and the capacitive gating effect, respectively. Comparatively, the capacitive gating effect needs stronger electrical fields excitation and can significantly modulate the graphene channel in a longer time. Utilizing the carrier trapping effect and the ambipolar property of graphene, the fundamental potentiation and depression behaviors can be emulated in each response mode. Then, utilizing the capacitive gating effect, the reconfiguration between excitatory and inhibitory response modes can be achieved. All synaptic responses only depend on the signals inputted through the back‐gate electrode, which is distinctively different from previous dynamic devices with additional modulating terminals. Such reconfiguration feature provides the artificial synapse the ability to emulate some complicated biological behaviors in future artificial intelligence systems, such as the adjustable perception of different external stimuli under different conditions. [ABSTRACT FROM AUTHOR]

Large-area and high-quality graphene grown on metal by chemical vapor deposition (CVD) has great potential applications in electronics and photo electronics. After more than 10 years of development, great progress has been made in the growth of graphene, and the role of various factors in the growth of graphene has been gradually clear. In this review, the role of hydrogen and oxygen in the formation and elimination of carbon-based impurities and silicon oxide particles and in the formation, observation, and control of grain boundary, point defects, and wrinkles is introduced. The content focuses on the specific experimental methods, results, and mechanism. Finally, the challenges of hydrogen and oxygen in the study of graphene growth on metal are introduced. This review can increase the theoretical knowledge and experimental design ability related to hydrogen and oxygen in CVD graphene grown on metal. • The concept of carbon-based impurity, silicon oxide particle, grain boundary, point defect and wrinkle, and their effects on the CVD graphene growth on metal are introduced in detail. • The role and the mechanism of hydrogen and oxygen in the study of carbon-based impurities, silicon oxide particles, grain boundaries, point defects and wrinkles are introduced. • We systematically share our experience and suggestions on the research of impurities and defects in CVD graphene growth on metal. [ABSTRACT FROM AUTHOR]

Wafer-scale single-crystal graphene with strong antioxidation is fundamentally important for their applications in electronics and optoelectronics. Although significant progress has been achieved in the chemical vapor deposition (CVD) growth of graphene, the production of wafer-scale graphene with high crystalline and excellent oxidation resistance still remains a challenge. Here, we report the epitaxial growth of 6-inch single-crystal graphene on twinned Pt (111) films with in-plane rotation of 60°(T-Pt) by ambient-pressure CVD. Our results show that the CVD-grown graphene on T-Pt exhibits fast growth rate and ultrahigh stability under the high-temperature air condition (>500 °C). The density functional theory (DFT) calculations reveal that the twinned Pt(111) surface does not change the preferential orientation of graphene nucleation, leading to highly aligned graphene domains on the T-Pt substrate. Moreover, the edge growth of graphene cannot be limited by the Pt twin boundaries (TBs), which is responsible for the fast growth of graphene single crystals. This work provides a reliable route to produce wafer-size single-crystal graphene monolayers with excellent oxidation resistance and clarifies the oriented growth mechanism of graphene domains on twinned Pt substrate. Here, we report an oriented multi-nuclei growth strategy toward 6-inch single-crystal graphene on twinned Pt (111) films with in-plane rotation of 60° (T-Pt) by ambient-pressure chemical vapor deposition. The graphene on Pt grows fast and has better oxidation resistance than graphene on Cu substrate. The density functional theory (DFT) calculations reveal that Pt-twinned structure does not change the preferential orientation of graphene nuclei, resulting in highly oriented graphene domains on the T-Pt substrate. This work provides a reliable approach to produce wafer-size single-crystal graphene monolayers with excellent oxidation resistance and clarifies the oriented growth mechanism of graphene domains on T-Pt substrate. [Display omitted] [ABSTRACT FROM AUTHOR]

AB-stacked bilayer graphene (BLG) has wide application prospects in the field of optoelectronic devices. Chemical vapor deposition (CVD) is an important method used to prepare BLG, but no definite stacking orientation is observed between the bilayers. Besides AB stacking, a number of disordered stacking orientations that could seriously affect the performance of BLG also exist. In this paper, we report a method to control the stacking orientation of reverse-transferred CVD-BLG via high pressure environment. We found that the orientation of BLG could be changed from weak-coupling stacking to AB stacking by increasing the temperature of the solvent in the autoclave. MoS 2 grains grown on graphene confirmed the stacking orientation variation of BLG. Reductions in the distance between layers in BLG under a high-pressure environment were considered to explain the observed orientation change. Reverse transfer allows the easy modification of the stacking orientation of BLG. The proposed high-pressure assistant method introduces a new route to prepare AB-stacked BLG and could be used to adjust the stacking direction of other two-dimensional materials. Increasing the pressure can change the distance between the two graphene layers, the distance reduced from d1 to d2 and the original stacking balance is modified. The stacking orientations of BLG can be determined by analyzing the orientation of MoS 2 crystals grown in SLG and BLG regions. After high pressure treatment, the orientation of MoS 2 crystals on the monolayer and bilayer is consistent. Image 1 [ABSTRACT FROM AUTHOR]

Abstract For graphene grown on Cu substrate via chemical vapor deposition (CVD), numerous nanometer particles are distributed along the graphene grain boundaries or evenly on the circumjacent substrate surface. The particles form new nucleation centers of graphene or destroy the graphene membrane during growth. In order to clarify the origin of particles, the formation process was studied by etching graphene at high temperature with different atmospheric pressures. We demonstrated that the formation of the particles is closely related to the competition of hydrogen and oxygen during growth; we also confirmed that the main component of particles was SiO x by energy dispersive spectrometry (EDS) measurement in transmission electron microscopy (TEM). Finally, on the basis of the formation mechanism, we proposed efficient approaches to reduce SiO x particles that improve the quality of graphene during actual CVD preparation process. Graphical abstract Image 1 Highlights • The formation mechanism of SiO x particles is closely related to the competition of hydrogen and oxygen impurity during graphene growth. • Hydrogen etches quartz to produce silicon, and then oxygen impurity react with silicon to produce SiO x particles in the case of hydrogen significantly reduced in the system. • The simplest and efficient measure to reduce SiO x particles is to avoid the sharp drop of hydrogen concentration during CVD. [ABSTRACT FROM AUTHOR]

We explored an easy technique to transfer directly large-area graphene grown by chemical vapor deposition (CVD) onto flexible substrates through substrate heating at appropriate temperatures. Polyethylene (PE) films were selected as the flexible substrate. The critical factor to ensure successful transfer was the good contact between the substrate and graphene. By heating the PE films merely approaching their glass-transition temperature, film morphology did not change. The stickiness of PE films further enabled the van der Waal forces to ensure close interaction between the viscoelastic substrate and graphene. This method did not require an intermediate transfer membrane such as polymethylmethacrylate (PMMA) that needs removal afterwards. Though this transfer method, monolayer and multilayer graphene films were transferred onto PE substrates with sheet resistance as low as 158 Ω/sq. All these results can be applied in existing and new graphene CVD applications on flexible substrates. [ABSTRACT FROM AUTHOR]

Single-crystal few-layer graphene films have potential applications in microelectronic and optoelectronic devices. Considerable efforts have been taken to prepare large-area bilayer graphene through chemical vapor deposition, but the growth of uniform graphene films with increased layers remains challenging. Herein, a hydrogen-modulation method for growing wafer-scale few-layer single-crystal graphene through chemical vapor deposition was proposed. By controlling the hydrogen flow rate at different stages, the dissolution amount and segregation process of carbon atoms in a CuNi substrate can be regulated separately. The hydrogen-modulation method fully exploits the effect of the dissolution–segregation mechanism, and the number of graphene layers can be controlled by modulating the hydrogen flow rate. Through the proposed method, the coverages of bilayer and trilayer graphene can reach more than 99% and 95%, respectively. The stacking and uniformity of few-layer graphene were evaluated by Raman and microstructure characterization, and bilayer and trilayer graphene were confirmed as AB and ABA stacks, respectively. Transmission line test structure and field effect transistor were prepared to verify the electrical properties of bilayer graphene. Results show that the square resistance of bilayer graphene is significantly lower than that of single-layer graphene, and it can also achieve high on-state current on field effect transistor. [Display omitted] [ABSTRACT FROM AUTHOR]

• The etching effect on graphene of hydrogen and oxygen was compared. • The etching effect of mixture of hydrogen and oxygen is weaker than that of oxygen. • The mechanisms of the etching effect of hydrogen and oxygen are different. Hydrogen and oxygen are the most commonly used gasses in the growth of chemical vapor deposition (CVD) graphene, which are very important for the growth of CVD graphene. In this work, a series of graphene etching and multistep growth experiments are designed to clarify the etching effect of hydrogen and oxygen to graphene on Cu. The etching effect to graphene in Ar (with oxygen impurities), mixture of H 2 and Ar, and growth atmosphere with different hydrogen concentration are studied in detail. The role of hydrogen and oxygen in graphene growth is further discussed finally. [ABSTRACT FROM AUTHOR]

Deposition of high quality dielectrics onto graphene is of significant challenge because of the chemically inert surface of the two-dimensional carbon lattice which cannot provide nucleating sites for high- κ oxide insulators. Here we utilize an ultrathin hydroxyl-free organic film, the benzocyclobutene (BCB) polymer, as a buffer layer between graphene and high- κ dielectrics in top-gate graphene transistors. By electrical measurements and Raman spectra, it has been demonstrated that the BCB-buffered dielectric stack does not significantly impact the electrical characteristics and doping level of graphene channel, allowing for high carrier mobility to be maintained in top-gate operation. Moreover, the application of the hydroxyl-free buffered dielectrics could remarkably decrease the density of interface state, and then suppress the hysteresis in graphene transistors, which is desirable for high performance graphene electronics. [ABSTRACT FROM AUTHOR]

Abstract: Hexagonal single-crystal graphene domains were grown on copper (Cu) foil via chemical vapor deposition and were etched with hydrogen at 950°C from 7 to 60min at atmospheric pressure. Numerous trenches were observed on the initial graphene domains after etching, and the trench patterns were closely associated with the Cu crystal orientation. No trenches were found if the etching process was conducted before cooling down. Thus, the etching trenches were bound up with the wrinkles formed during the cooling down process. Then, the process of etching on the wrinkles was examined. This simple hydrogen etching technology proved that wrinkles and point defects existed even in hexagonal single-crystal graphene domains. This method could be a convenient way to detect the distribution and morphology of wrinkles in graphene. [Copyright &y& Elsevier]

[Display omitted] • A transfer-free strategy to synthesize graphene films on dielectrics is developed by using 6H-SiC and molten gallium (Ga)-palladium (Pd) alloy as precursor and catalyst. • The Ga-Pd catalyzed interfacial growth method integrates the advantages of catalytic growth of graphene on metals and epitaxial growth on silicon carbide (SiC) by thermal decomposition. • The catalytic Ga-Pd alloy induces the decomposition of Si C bonds and promotes the fabrication of graphene films on the interface between molten alloy and SiC substrate as well as the upper surface of alloy. Direct growth of graphene on insulating substrate is highly desired for practical applications in two-dimensional electronics and optoelectronics. However, the controllable synthesis of large scale graphene films on dielectrics is still limited in thickness and crystallinity by the absence of catalyst. Here we develop a transfer-free method to synthesize continuous graphene films on various insulating substrates by using molten Ga-Pd alloy and silicon carbide (SiC) as catalyst and carbon source. The catalytic Ga-Pd alloy induces the decomposition of Si C bonds and promotes the formation of C-C sp2 at relatively low temperature around 1000 °C. The suitable carbon solubility of Ga-Pd alloy could also promote the fabrication of few-layer graphene films on the interface between alloy and SiC substrate as well as the upper surface of alloy. Furthermore, this strategy could be extended to the interfacial growth of graphene on other dielectrics by simply placing the substrate face down to the surface of molten alloy. The progress of this work may help to understand the mechanism of molten Ga-Pd alloy catalyzed interfacial growth of transfer-free graphene and benefit the application of graphene-based electronics and photonics in the future. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the synthesis of high quality graphene on single-crystal Mo(110) films by chemical vapor deposition (CVD) of methane was reported for the first time. X-ray diffraction proves that carbon species had dissolved into the metal. The micro-Raman spectroscopy indicates that the thickness of Mo films, cooling rate and growing time play significant roles in the quality of as-grown graphene films. By optimizing the growth time (15min) and cooling rate (10°C/s), we achieved high quality graphene films with the small ratio I G/I 2D≈0.26 and the FWHM(2D)≈30.4cm−1 on 200nm-thick Mo films. Our experiments also suggest that graphene growth on Mo is a dissolution and segregation process as Ni. [Copyright &y& Elsevier]

• A simple synthesis method of scalable adlayer-free monolayer graphene on CuNi foil. • Graphene grown on Cu 80 Ni 20 has the advantage of much wider growth window than Cu. • Proposing a growth model of adlayer-free monolayer on Cu 80 Ni 20 substrate. Homogeneity is crucial to material applications. Although monolayer graphene was successfully prepared more than a decade ago, to date, the growth of adlayer-free monolayer with a large area remains challenging. Here, we report the synthesis of metre-size adlayer-free monolayer graphene film on Cu 80 Ni 20 foil via atmospheric pressure chemical vapour deposition. Experimental results show that the growth parameter window of adlayer-free monolayer graphene on Cu 80 Ni 20 is very wide, which makes the growth of adlayer-free monolayer graphene easy and controllable. A growth model of adlayer-free monolayer graphene on Cu 80 Ni 20 substrate is also proposed. [ABSTRACT FROM AUTHOR]

Recently, a hybrid graphene/poly-BrNpA phosphor structure was used to fabricate rewritable p-n junction photodiode responsive to visible light. In this process, photoisomerization occurred under moderate ultraviolet (UV) irradiation (λ = 365 nm, optical intensity I UV = 1.318 μW μm−2), and the subsequent water (H 2 O) molecule adsorption facilitate charge transfer at the interface. In this study, we explore the feasibility of making UV photodetector based on the hybrid graphene/poly-BrNpA field-effect transistor. The incident UV light (λ = 365 nm, I UV ≤1.81 × 10−4 μW μm−2), which is at least four orders weaker than that used to induce observable photoisomerization in the phosphor, is mostly absorbed to create photo-induced electron-hole pairs. The subsequent separation and transfer of the photo-induced electrons to graphene contribute to high photocurrent by the photogating effect. The photodetector has a responsivity and specific detectivity up to 159 A W−1 and 1.55 × 1011 Jones, respectively, when biased at V DS = 1 V. The photodetector has a slow response, especially to UV shut-off with the fall time τ F up to 881 s. By virtue of H 2 O molecule adsorption, τ F can be reduced to 6.6 s by accelerating the reversion and recombination of electrons with the photo-induced holes trapped in the poly-BrNpA film through introducing defect states. This work brings a new perspective for developing photodetectors through combining the functional two-dimensional materials and phosphors. • UV photodetectors were fabricated in the configuration of hybrid graphene/phosphor structure. • High responsivity and high detectivity were obtained. • The decay time of the photodetector could be shortened by two magnitude by water adsorption. [ABSTRACT FROM AUTHOR]